Enable effective debugging

[sehilyi]

Jan Aerts from BioVis asked about how one can debug in Gosling.js.

Since it is vega-like: how can you debug the incremental construction of visuals. It is always challenging to debug in Vega.

This is a great point that we will need to explore.

Currently, we show log messages to show whether the schema is valid and what is the issue if any (e.g., "x_1 is not valid property" in the Editor).